In aldol condensations, a compound having an alcohol and an aldehyde or ketone group is synthesized from the condensation of two aldehydes, two ketones, or a ketone and an aldehyde. Such reactions are commercially important in the production of intermediates needed to synthesize other commercially important products. Further, many of the aldehydes and ketones that result from application of aldol processes are attractive for use as solvents due to the significant polarity they exhibit.
Unfortunately, aldol condensation reactions have shown a proclivity for nonspecificity. Typically, numerous competing reactions occur creating a large array of cross reacted products. Many of these cross aldol products can be further engineered to ultimately produce either the product that was initially sought or some other desirable product. However, even if useful products can be made from these cross reacted species, their presence complicates batch processes. Moreover, a greater amount of feed stock is necessary to obtain final product from a process which produces cross reacted products.
Each cross reacted species represents added cost to the production of solvents because more varied and numerous equipment is required. This includes additional distillation columns and reactors, additional reagents, and additional catalysts. This is particularly true in one step aldol processes. The numerous products that result from these reactions are frequently of similar molecular weight, polarity, and boiling point. This makes separation extremely difficult and makes commercialization of these processes particularly troublesome and expensive.
Methyl Isobutyl Ketone (4-methyl-2-pentanone, hereafter MIBK), has been a particularly important solvent produced by a combination of aldol condensation, dehydration, and selective hydrogenation reactions. MIBK has been extensively used as a solvent in coatings, paints, rare metal extractions, as a denaturant, and in other applications.
Environmental concerns have recently prompted the search for effective alternatives to MIBK. Methyl n-amyl ketone (MNAK) and several other ketones have been found to have many of the same desirable characteristics as MIBK with a lessened environmental impact. This has caused a surge in the demand for such ketones that is likely to continue to grow. Supplying this demand for MIBK replacements presents a significant challenge.
The development of safe and efficient processes for the commercial production of MIBK has been one of the resounding success stories of the chemical industry. MIBK production process can be conducted in three steps. First, acetone (dimethyl ketone, hereafter "DMK") is treated with an alkali catalyst to produce diacetone alcohol (hereafter "DAA"). This DAA is then acid-catalyzed to dehydrate into mesityl oxide (hereafter "MO") which is selectively hydrogenated over a Nickel or Copper containing catalyst to produce MIBK.
Many of the facilities producing MIBK currently in use have been operating for several decades. Of course, efforts aimed at improving the economics of MIBK production and ketone production in general have been continuous. A great deal of effort in this regard has been applied to the development of one-step reactions. In the typical one-step process for the production of MIBK, a mixture of acetone and hydrogen are reacted in the presence of a catalyst system comprised of a zeolite impregnated with a metal such as palladium or nickel to produce the end product ketone. Other catalysts and catalyst systems have been proposed but most involve fairly specialized chemistries. Further, difficulties are encountered in separating and refining products of these one-step reactions as noted above. The limitation of cross reactivity to a manageable number of separable products is due to the specificity of the catalyst used in these reactions.
Examples of one-step aldol processes include British Patent 1,252,335 to Sumitomo Chemical Co. This patent is drawn to a one-step vapor phase aldol condensation of acetone to obtain MIBK. U.S. Pat. No. 4,866,210 to Hoelderich, et. al. is drawn to a one-step reaction for producing unsymmetrical (aliphatic ketones by an aldol reaction catalyzed by an acid catalyst supported by a zeolite or aluminum phosphate carrier. U.S. Pat. No. 4,102,930 to Deem is drawn to a one-step aldol reaction for producing ketones and aldehydes in which a particular catalyst is claimed. U.S. Pat. No. 4,701,562 to Olson is drawn to a one-step process for condensing aldehydes catalyzed by a nonzeolitic aluminophosphate. U.S. Pat. No. 5,055,620 to Letts is drawn to a one-step aldol condensation reaction in which a pseudoboehmite borne catalyst is employed.
U.S. Pat. No. 4,739,122 ('122) to Letts proposes a cross-aldol condensation for producing Methyl Amyl Ketone (MAK) using a hydrogen reduced copper oxide on gamma alumina catalyst. The product of the process is a mixture of closely related ketones many of which appear in extremely small quantities. "Useful products" in this patent are defined as a mixture of at least four different ketones (MAK, Ethyl Hexaldehyde, Diamyl Ketone, and MIBK). At least three components in this mixture are byproducts that are produced in small quantity (Ethyl Hexaldehyde, Diamyl Ketone, and MIBK). U.S. Pat. No. 4,239,657 ('657) to Nissen, et. al. proposes a one-step process for making aldehydes and ketones of not less than four carbons. A cobalt/nickel/zinc oxide/metallic catalyst on a support is claimed with great particularity. The processes disclosed in the '122 and '657 patents produce, in addition to MIBK, certain products which are referred to throughout this specification as ketone and aldehyde byproducts. These ketone and aldehyde byproducts include 4-methyl-3-penten-2-one; 3-heptene-2-one; undecan-6-one; 4-undecen-6-one; 4,7-undecadien-6-one; and butyl butyrate. In the processes proposed, these chemical species are largely unusable yield losses.
A number of patents are directed solely to catalysts used to improve the selectivities and yields of aldol condensations. U.S. Pat. No. 4,165,339 to Reichle describes a complex catalyst system for improving the performance of known aldol condensation processes particularly those that produce mesityl oxide and isophorone from acetone; U.S Pat. No. 4,146,581 to Nissen, et. al. describes a catalyst system for producing higher ketones; U.S. Pat. No. 4,270,006 to Heilen, et al. describes a catalyst system largely incorporating noble metals and salts of rare earth metals; U.S. Pat. No. 4,049,571 to Nissen, et. al. describes another catalyst for one-step aldol condensations that yield higher ketones; U.S. Pat. No. 4,086,188 to Reichle describes such a catalyst comprised of an alkaline metal or alkaline earth metal and a doped solution of water with a zinc or lithium salt while his U.S. Pat. No. 4,476,324 is directed to a heat treated anionic clay mineral for the same purpose.
Selective catalysts can generally be used to produce either MIBK or MNAK. Producing both concurrently in the same reaction vessels or production setups according to one-step processes has not been commercially successful. Further, since aldol condensations generally produce a complex mixture of cross reacted products, separating such aldol products would be expected to involve process engineering that is specific to the desired product. Even if one could successfully produce both MIBK and MNAK simultaneously, the process redesign and additional new separation technologies required would be substantial and difficult.
While the demand for MNAK may surpass that of MIBK, it will probably not supplant it entirely. Numerous applications for which MIBK is particularly well suited will continue for years to come. Dedicating time, effort, and resources to building MNAK production facilities from existing MIBK facilities or simply establishing separate processes may create a Hobson's choice for those who wish to produce both MNAK and MIBK. They must either invest substantial capital in the production of new and separate production facilities or they must reequip their MIBK facilities to such a degree that MIBK can no longer be produced in commercially meaningful quantities. This could quickly make the production of MIBK replacement ketones an economically unattractive proposition for a manufacturer.
A method of producing high value ketones such as MNAK from existing MIBK production facilities would provide an economic means for producing both MIBK and MNAK. Such a process employing easily obtainable catalysts without the need for extensive separation steps would be more desirable yet. Moreover, coproduction of such high value ketones would greatly reduce duplication of effort and provide manufacturers flexibility in ketone production.